Persistent Photoinduced Changes in Charge States of Donors and Acceptors in Hydrothermally Grown ZnO
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0957-K03-04
Persistent Photoinduced Changes in Charge States of Donors and Acceptors in Hydrothermally Grown ZnO N. C. Giles, Yongquan Jiang, Xiaocheng Yang, S. M. Evans, and L. E. Halliburton Physics Department, West Virginia University, Morgantown, WV, 26506
ABSTRACT Bulk ZnO grown by the hydrothermal technique was investigated using electron paramagnetic resonance (EPR), photoluminescence (PL), and infrared absorption (FTIR) techniques. Isolated substitutional lithium is the dominant acceptor and could be detected using EPR or PL. A large concentration of neutral Li+-OH− centers were observed using FTIR data. EPR spectra assigned to Mn, Co, Ni, Fe, and Group III (Al, Ga) donors were also observed. Photoinduced changes in the charge states of the different deep and shallow centers were produced using 325 nm light, and the stability of these changes were monitored with EPR during pulsed thermal anneals. The charge-state changes for some defects were persistent and remained up to 300 K. These impurities, when present in device structures, may act as stable charge trapping sites. INTRODUCTION Lithium is the dominant electrically active acceptor found in most hydrothermal (HT) bulk ZnO. The Li ions come from the mineralizer added to the growth solution. EPR from neutral lithium acceptors has been reported [1]. A detailed study [2] showed that an isolated Li ion, substituting for Zn, is a deep acceptor. The hole is localized on one of the adjacent oxygen ions, and the accompanying lattice distortion contributes to the deep nature of this acceptor. Many of the Li acceptors in hydrothermal ZnO are passivated by hydrogen in the form of OH− ions [3], thus forming a neutral stable Li+-OH− complex. This complex is electrically inactive and can be monitored with infrared absorption. In addition to the Li impurities, many donors (shallow and deep) are present in HT ZnO. Group III impurities (Ga, Al) act as effective-mass donors and their EPR is well known. PL measurements of bound-excitons provide another means to detect these shallow donors. Transition-metal (TM) ions are incorporated on the zinc site in ZnO and are present in many undoped bulk ZnO crystals. TM impurities act as deep donors, and are normally divalent in n-type crystals (i.e., as Fe2+, Ni2+, Co2+, and Mn2+). In crystals with lower Fermi levels, the trivalent charge states are present when these donors provide compensation for acceptors. An ionic notation is used to describe the charge states: a substitutional TM2+ ion is a neutral donor (D0) and a TM3+ ion is a singly ionized donor (D+). EPR is a sensitive and direct method to monitor isolated TM ions in ZnO crystals. Numerous EPR studies of isolated Mn2+, Fe3+, Co2+, and Ni3+ ions in ZnO have been reported [4-7]. Minimal random strain and a lack of superhyperfine interactions (67Zn is 4.1% abundant and 17O is 0.038% abundant) give narrow lines (< 1 G width) and allow small concentrations of TM ions to be observed.
Photoinduced changes in charge states of shallow donors, TM donors, and Li acceptors in a HT
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